1. Field of the Invention
The present invention relates to barrels used in injection molding and extrusion processes and more particularly to bimetallic barrels having alloy inlays. Injection and extrusion processes are widely used in the plastic, chemical, food and ceramic and metal powder industries.
2. Description of the Related Art
In injection molding and extrusion processes, solid plastic resin is heated and liquefied inside a hollow cylinder or barrel by heater bands that envelop the cylinder and by the frictional heat generated by the rotational action of the screw. The molten plastic is discharged from the cylinders and enters a mold. The molten plastic solidifies in the mold to form various objects.
During the plasticization process, the plastic resin will wear against the bore of the cylinder. Besides, the plastic often contains various fillers which are highly abrasive. Also, aggressive chemicals generated during the process can corrode the bore. For instance, fluoropolymer, polyvinyl chloride, and resins containing flame retardants will generate hydrofluoric or hydrochloric acid, which are very corrosive. If the bore of the cylinder is enlarged due to wear or corrosion, the efficiency of the process will decline.
The injection pressure for a plastic injection molding machine is typically between 20,000 and 30,000 psi and could be as high as 37,000 psi. Barrels for plasticizing resins not only have to contain the high pressure but also have to resist corrosion and wear of the bore from the resin.
Alloy steels such as 4140, 6150, and 8620 are often used as backing steels for bimetallic barrels. Their disadvantages are discussed below.
Some barrels are made of heat treated and nitrided steel. These barrels have good strength, but their wear resistant nitrided surface is very thin, usually 0.005. to 0.010 inch. Once this thin nitrided layer is worn out, accelerated wear will follow. Moreover, the nitrided steel has poor corrosion resistance to various corrosive resins.
A more advanced barrel is made by a centrifugal casting process. A corrosion and wear resistant alloy is cast in the bore of a steel cylinder to form an inlay by centrifugal casting. The inlay is typically 0.060 inch to 0.100 inch thick and is metallurgically bonded to the steel cylinder bore. These bimetallic barrels have been very successful in processing various corrosive and abrasive resins.
Bimetallic barrels having an inlay alloy containing nickel, boron, silicon, tungsten, carbon and chromium are widely used. The inlay alloy, designated (A), has tungsten carbide particles embedded in a nickel alloy matrix. The chemical composition is as follows:
______________________________________ C Si B Cr Fe Ni Co W ______________________________________ (A) 1.3 0.5 1.0 5.0 25 Bal 3.0 25.0 to to to max max to to 3.5 3.3 3.0 7.0 50.0 ______________________________________
However, bimetallic barrels cannot be strengthened by a heat treatment process because the quench process in the heat treatment will crack the hard and brittle inlay. In the manufacturing of bimetallic barrels, generally steel will lose its strength during the slow cooling which is required to prevent the inlay from cracking. In order to contain high pressures, bimetallic barrels are often sleeved with a strong, heat treated steel sleeve on the outside of the high pressure end of the barrel. The sleeve is welded to the barrel. However, barrel failures during injection molding are often attributed to breakage of the weld or collapse of the inside liner.